A liquid developer for electrostatic photography generally comprises a liquid carrier having a high electric resistance (from 10.sup.9 to 10.sup.15 .OMEGA..cm), positively or negatively charged toner particles, a fixing resin for providing a fixing ability to the toner particles, a dispersing agent for stably dispersing the toner particles, a charge controlling agent for positively or negatively charging the toner particles and various additives. The charge controlling agent provides a definitely positively or negatively charged polarity to the toner particles, and controls the amount of the charge, and, thus, the agent is an essential ingredient for the liquid developer.
When the toner particles are composed of a resin which acts as a fixing resin, charge controlling can be roughly divided into two methods. The first method comprises introducing an ingredient for providing chargeability to the toner particles and receiving ions by adding a charge controlling material to generate a charge. The known charge-providing components used for this purpose include monomers containing tertiary amines or quaternary ammonium salts (for example, dimethylaminoethyl methacrylate and octyl tosylate of dimethylaminoethyl methacrylate). The known charge controlling materials include metal salts of naphthenic acid such as nickel naphthenate and cobalt naphthenate; metal soaps such as cobalt 2-ethylhexanate; metal salts of sulfonic acid such as calcium dodecylbenzenesulfonate, metal salts of petroleum sulfonic acid and metal salts of sulfosuccinic acid esters; lecithin; polyvinylpyrrolidone resins; polyamide resins; sulfonic acid-containing resins as described in JP-B-56-24944 (the term "JP-B" as used herein means an "examined Japanese patent publication"); hydroxybenzoic acid derivatives as described in JP-A-57-139753 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"); metal salts such as a nickel salt of amino acid derivatives as described in JP-A-61-50951; semi-alkylamide compounds of diisobutylene-maleic acid copolymers as described in JP-B-49-26596; semi-maleic acid amides as described in JP-A-60-173558, JP-A-60-179750 and JP-A-60-182447; and itaconic anhydride derivatives.
This method, however, has a problem in that introduction of the charge-providing component greatly affects grain sizes and polymerization reactivity of resin grains when the resin grains are formed in the presence of a dispersion stabilizing resin which is dissolved or dispersed in a colloidal form in an non-aqueous solvent. Thus, the amount of the charge-providing component is markedly limited thereby resulting in the difficulty of controlling the charge amount.
The second method comprises coating the toner particles with a material having a component for providing chargeability (which may be the dispersion stabilizing resin) and transferring ions by adding the charge controlling material to generate charge. The charge-providing components include, for example, soluble quaternary ammonium copolymers as described in JP-A-54-31739, JP-A-59-137960 and JP-A-61-39059. The charge controlling materials include the same materials as used in the first method described above. This method has an advantage of exerting less influence on the polymerization reactivity as compared with the first method, and particularly making it possible to introduce a quaternary ammonium salt. However, the dispersing agent into which a quaternary ammonium salt has been introduced has disadvantages in that solvents used for the synthesis of resins is largely restricted because of its amphiphilicity, i.e., both the hydrophilicity and the lipophilicity, and that the use of the quaternary ammonium salt in polymerization reaction is largely restricted because of its insufficient solubility in non-aqueous solvents.